Many processes are known for separating components from a fluid mixture. Chromatography makes use of a wide range of adsorbents having varying degrees of affinity for different components of a fluid mixture, thereby causing the components to separate as they flow through the adsorbent. Similarly, materials known as molecular sieves may affect the rates at which components of a fluid mixture pass through them by admitting only molecules of certain components into the pore structure of the material, but not of other components. Some adsorbents are also believed to provide separation by differences in the electrochemical attraction of the components. Thus, the component for which the adsorbent has the greater affinity or retention capacity may be recovered or desorbed by means of a desorbent material.
Processes for separating components from a fluid mixture based on the use of adsorbents for chromatographic type separations include simulated moving bed (SMB) systems. The flexibility of SMB technology is well established as many combinations of adsorbents, desorbents, equipment configurations, and process conditions are well known for a wide variety of industrial applications. Examples of such processes are: the separation of linear paraffins from branched-chain and cyclic hydrocarbons, olefins from paraffins, para-cresol or meta-cresol from cresol isomers, para-cymene from cymene isomers, 1-butene from a mixture of paraffins and olefins which contain four carbon atoms, fructose and glucose from mixtures thereof, ethylbenzene from xylene isomers, cyclic hydrocarbons and olefins from paraffins, isomers of C8 aromatic hydrocarbons from mixtures thereof, chiral compounds for use in pharmaceuticals and fine chemicals, oxygenates such as alcohols and ethers, and the gas phase separation of propylene from propane.
It is also known that the composition of the extract and raffinate streams withdrawn from a SMB adsorptive separation circuit fluctuate when the non-selective volume is unevenly distributed in the circuit. The term “discrete non-separating section” herein refers to any portion of non-selective volume that is non-uniformly distributed in the circuit. This term is discussed and defined in more detail later herein. A common example is the conduit and associated equipment that connects the two ends of a column containing the adsorbent where all the adsorbent is loaded in a single column.
U.S. Pat. No. 5,582,736 is directed to a SMB adsorptive separation process characterized in that each time an extract or raffinate stream passes from a position immediately anterior to a position immediately posterior to each of the dead volumes in the circuit, the flow rate regulated by the recycling pump is increased during the time when the extract or raffinate remains in the position immediately posterior to a dead volume and then reducing the flow rate to the value which would have been applied if the dead volume had been ignored when the extract or raffinate passes from the position immediately posterior to the dead volume to the following position.
U.S. Pat. No. 5,578,215 is directed to a SMB adsorptive separation process having independent shifting of the injection and extraction circuits. The process is characterized in that the time an injection or extraction stream is connected to the circuit is increased each time the stream moves from a position immediately anterior to a position immediately posterior to each dead volume or degraded section and then reducing the connection period to the value which would have been applied if the dead volume or degradation had not occurred when the injection and extraction stream passes from the immediately posterior position to the following position.
U.S. Pat. No. 5,578,216 is directed to a SMB adsorptive separation process having a dead volume in the circuit. The process is characterized in that the volume of the section immediately upstream of the dead volume is reduced by an appropriate value when the dead volume is located downstream of this section and upstream of the extraction streams of the section, or the volume of the section immediately downstream of the dead volume is reduced by an appropriate value when the dead volume is located downstream of the injection streams into the section and upstream of this section.
U.S. Pat. No. 6,063,285 is directed to regulating the flow rate of at least one fluid in a SMB adsorptive separation process through the use of at least one restriction means in addition to the control valve in the pressure controlled withdrawal line enabling the pressure downstream of at least one of the beds to be reduced.
U.S. Pat. No. 5,102,553 is directed to controlling flow rates through individual compartments in a SMB adsorptive separation process in accordance with time variable functions in each step.